The specific aims of this project are: 1. To determine the activity of lysosomal endopeptidases (cathepsins) along the rat and rabbit nephron and to compare the activity of these enzymes with the size and ultrastructure of the lysosomal compartment in the same nephron segments. 2. To determine the capacity of the lysosomal system in individual nephron segments to digest proteins of different molecular size and charge including albumin, myoglobin, lysozyme and cytochrome C. 3. To examine the effect of proteinase inhibitors, lysosomotropic weak bases, and a basic amino acid on a) The activity of lysosomal endopeptidases and the size and ultrastructure of the lysosomal-vacuolar compartment in proximal and distal nephron segments of the rat kidney; b) The degradation of anionic and cationic protein in individual nephron segments; c) The uptake and intracellular distribution of protein in the rat proximal tubule using horseradish peroxidase and ferritin as tracer proteins in combination with transmission electron microscopy. These studies require the use of various biochemical and morphologic techniques such as microdissection of individual nephron segments, ultramicroassays of various enzymes, radioactive labeling of proteins, measurements of protein degradation by liquid scintillation counting, micropuncture of kidney tubules, transmission electron microscopy, light microscopy, ultrastructural tracer techniques, and ultrastructural histochemistry. The long term objective is to continue earlier investigations by the P.I. involving the structure and function of the lysosomal system in the kidney. This proposal emphasizes the role of the lysosomal-vacuolar system in the uptake, intracellular transport, and degradation of proteins of various size and charge in the various segments of the mammalian nephron.